Plasma arc torches are widely used for cutting metallic materials and can be employed in automated systems for automatically processing a workpiece. The system may include the plasma arc torch, an associated power supply, a positioning apparatus, and an associated controller. The plasma arc torch and/or the workpiece may be mounted on the positioning apparatus which provides relative motion between the tip of the torch and the workpiece to direct the plasma arc along a processing path.
The plasma arc torch generally includes a torch body, an electrode mounted within the body, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, a nozzle with a central exit orifice, electrical connections, and a power supply. The torch produces the plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. A shield may also be employed on the tip of the torch to protect the nozzle and to provide a shield gas flow to the area proximate the plasma arc. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, the tip of the torch is positioned proximate the workpiece by the positioning apparatus. A pilot arc is first generated between the electrode (cathode) and the nozzle (anode) by using, for example, a high frequency, high voltage signal. The pilot arc ionizes gas passing through the nozzle exit orifice. As the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc transfers from the nozzle to the workpiece. The torch is operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, to cut the workpiece.
The controller directs the torch tip along a nominal processing path. Due to variability in workpiece geometry and relative spatial location of the workpiece and the torch tip, the standoff or distance between the torch tip and workpiece may vary along the processing path. When employing a constant output current generator, changes in standoff and resultant plasma arc length effect arc voltage, arc power, and cut quality. In extreme cases, the torch tip can crash into the workpiece or be so far from the workpiece that the plasma arc is extinguished.
Some systems employ feedback control, controlling the arc voltage during processing by adjusting the standoff to maintain a predetermined arc voltage value. Such control schemes, however, are problematic in a variety of common scenarios. For example, when the torch tip traverses a discontinuity in the workpiece such as a kerf, the arc voltage increases rapidly. To compensate for the increase, the controller directs the positioning system to decrease standoff rapidly, which can result in the torch tip crashing into the workpiece. Such shortcoming limits the usefulness of these feedback control schemes.
Other types of feedback control are known to initially position the torch tip relative to the workpiece. For example, some systems drive the torch tip into the workpiece until the structure on which the torch is mounted deflects. Corrective action is then taken, such as reversing direction of travel to retract the torch a predetermined distance. Other systems may drive the torch tip into the workpiece until detection of an increase in drive motor power, then reverse motor direction for a predetermined period. These systems, however, tend to stress the mechanical components of the positioning apparatus and torch and can damage delicate components thereof due to repeated impact.